Method for production of sodium bicarbonate, sodium carbonate and ammonium sulfate from sodium sulfate

ABSTRACT

There is disclosed a process for recovering sodium bicarbonate and forming ammonium sulfate from a source containing sodium sulfate. The method involves contacting the sodium sulfate in solution with carbon dioxide and a compound containing ammonia. Sodium bicarbonate is precipitated in high purity form from the solution. It is important to maintain the temperature of the source solution at or above 32° C. This provision eliminates contamination of hydrates or ammonium bicarbonate components. The filtrate of the sodium bicarbonate reaction can be further processed to yield an ammonium sulfate product in the concentrated liquid or precipitated form in high purity. The basic process can be expanded to be combined with a conventional Claus process for sulfur recovery as a Tail Gas Unit, or can be further employed in a wet and dry scrubbing process for FGD schemes.

This is a continuation-in-part of U.S. application Ser. No. 08/547,773,filed Oct. 10, 1995, now U.S. Pat. No. 5,830,422 which is acontinuation-in-part of U.S. application Ser. No. 08/494,073 filed Jun.23, 1995, now U.S. Pat No. 5,654,351.

FIELD OF THE INVENTION

The present invention relates to a process for generating sodiumcarbonate compounds and ammonium sulfate and more particularly, thepresent invention relates to a process for generating theabove-mentioned compounds in a substantially pure form from gas streamscontaining sulfur oxides such as flue gas streams.

BACKGROUND OF THE INVENTION

There have been numerous processes previously proposed for themanufacture of alkaloid carbonate, various sulfates, and thedesulfurization of flue gases. One of the primary difficulties with theknown procedures for manufacturing, for example, sodium bicarbonate andammonium sulfate is the fact that a pure product is difficult to obtainwhen one employs the methods previously set forth in the art.

Typical of the previously proposed methods in this art is U.S. Pat. No.3,846,535, issued Nov. 5,1974 to Fonseca. The Fonseca reference teachesa method for absorbing sulfur oxides from gaseous mixtures containingsulfur oxides. A sodium bicarbonate or potassium bicarbonate absorbentis employed to achieve this result and the absorbent is regenerated. TheFonseca reference delineates how calcium sulfate may be produced forwallboard, landfill and other such products, but fails to teach a methodfor producing either pure sodium bicarbonate or ammonium sulfate inpurity levels suitable for commercial sale and further for flue gasdesulfurization. The Fonseca disclosure indicates that only 67% of thetheoretical amount of sodium bicarbonate obtained has an 87% puritytherefore leaving a continuous process imbalance without external sodiumbicarbonate supply.

A further limitation in the Fonseca reference is that there is noteaching concerning a source of ammonium ions to enhance theeffectiveness of the process and result in desirable production ofammonium sulfate and sodium bicarbonate.

Demonstrative of the imprecise teachings in the reference regarding theprocedure to achieve the most desirable process and quality products isthe passage in column 3 beginning at line 52, wherein it is indicated:

"The aqueous ammonium sulfur oxide mixture produced by the precipitationof sodium bicarbonate can optionally be oxidized and recovered asammonium sulfate. The ammonium sulfate is useful as a fertilizer and fornumerous other industrial purposes."

If this were practiced, there would be a deleterious effect to theoverall directive of producing sodium bicarbonate and ammonium sulfate.

Further prior art in this file includes that taught in Canadian PatentNo. 543,107, issued Jul. 2, 1957, to Downes. The reference teaches amethod of separating polybasic acids from their aqueous solutions andthe recovery of ammonium sulfate from aqueous solutions. The disclosureindicates that the treatment of sodium sulfate for the production ofsodium bicarbonate and ammonium sulfate may be achieved by exposing theaqueous solution of sulfate to ammonia and carbon dioxide. The result isthe precipitation of sodium bicarbonate. Although the Downes method isuseful to recover the sodium bicarbonate, there is no teaching in thedisclosure concerning how an uncontaminated product of sodiumbicarbonate and ammonium sulfate, since these are reciprocal salt pairscapable of the formation of a double salt which can be produced byfollowing the method. In addition, the method as set forth in thisreference would appear to be susceptible to the formation of hydratesone being known as Glauber salt when using these salt pairs.

Stiers, in U.S. Pat. No. 3,493,329, issued Feb. 3, 1970, teaches amethod of making sodium carbonate. The Stiers method is aco-precipitation method and cannot provide for selective precipitationof desired products since the salts are reciprocal salts and form adouble salt. In the Stiers method, the desire is to remove the sulfateanion to use it for the transportation of sodium cations from sodiumchloride to the bicarbonating process as sodium sulfate. In addition tothe above, the Stiers process involves the continuous recycling of themother liquor which requires that the ammonium sulfate in the liquor becontinuously removed or reduced from the process stream. If the ammoniumsulfate reaches a saturation point in the bicarbonating stage, ammoniumsulfate will co-precipitate with the sodium sulfate in the form of adouble salt compound or two inseparable salts.

Stiers demonstrates a process to generate two salts and double saltsrather than a pure single salt, the lafter being much more desirablefrom a commercial point of view.

Canadian Application Serial Number 2,032,627, offers an innovativetechnique to produce the desirable pure products. This method employed anumber of evaporative and cooling techniques to alter the solubility ofsodium sulfate and ammonium sulfate in solution and selectivelyprecipitate the desired pure components. Lab bench scale batch testingof this method demonstrated effective results, however, continuous pilotscale testing clearly identified undesirable limitations to the processas specified. More specifically, the process is difficult to operate ina consistent and continuous mode and as such is highly susceptible toammonia sulfate contamination with sodium sulfate, resulting in acommercially undesirable double salt product.

In greater detail of the teachings of the Canadian application, it istaught that brine remaining after screening sodium sulfate has atemperature of 95° C. to 60° C., the solubility of ammonium sulfate isdecreased while the solubility of sodium sulfate increases. The resultis that more ammonium sulfate precipitates while keeping sodium sulfatein solution.

By following the teachings, the mixed solution is supersaturated withsodium sulfate due to evaporation at 95° C. and specifically results inthe production of double salt when the ammonium sulfate crystallizationstep of 60° C. is attempted as a continuous process.

In view of what has been previously proposed in the art, it is clearthat a need exists for a process of recovering sodium carbonatecompounds and the formation ammonium sulfate from a source of sulfatewhich overcomes the limitations regarding purity, precipitation,selectivity and other such limitations and reduces sulfur emissions fromindustrial facilities. The present invention is directed tocircumventing the previously encountered difficulties of reciprocatingsalt pairs and employing the improved process for flue gasdesulfurization and inorganic value recovery.

SUMMARY OF THE INVENTION

One object of one embodiment of the present invention is to provide animproved process for the scrubbing of various sulfur compounds from, forexample, flue or tail gas streams containing sulfur compounds.

Advantageously, the use of solubility data for sodium bicarbonate,ammonium sulfate and sodium sulfate provides the necessary informationfor effecting selective precipitation without the contamination of oneprecipitate effecting a further precipitate as was conventional with theprior art processes. By making use of a solubility data, it is possibleto precipitate sodium bicarbonate and ammonium sulfate withoutprecipitating sodium sulfate as a contaminant.

By controlling temperatures and pressures, once a bicarbonateprecipitate is formed, the filtrate may be subjected to a purificationstep wherein the remaining sodium ions are substantially removed or madeto be held in solution prior to the precipitation of ammonium sulfate.This results in a cleaner precipitate of ammonium sulfate and thereforeresults in a more commercially desirable product, which product exceedspurity measures not previously encountered with the prior art processes.In a purification possibility, the filtrate may be supersaturated withammonia in a conditioning reactor which operates at a substantiallycooler temperature, for example, 7° C. This is one example of anappropriate temperature, a suitable range is between about 20° C. toabout -40° C. This procedure results in the formation of sodium sulfateor a mixed salt of ammonium sulfate and sodium sulfate, both of whichare insoluble at these temperatures and this excess of ammonia. Onceprecipitated, the filtrate, therefore having a lower concentration ofsodium cations inherently leads to a less contaminated precipitatedammonium sulfate.

Several different conditioning processes are also possible which includesodium ion reduction by, for example, ion exchange techniques,refrigeration, evaporation, etc.

The process is further enhanced by providing an ammonia and carbondioxide chemical recovery scheme for minimizing the chemical consumptionto enhance commercial viability. It has been further found that bymaking use of the basic bicarbonate recovery process, that the processcan be used for additional fields of utility, for example, tail gasdesulfurization, flue gas desulfurization by wet and/or dry sorbentinjection techniques and further applications for making commercial orlandfill grade gypsum and fully recovering the ammonia chemical.

Flue gas desulfurization (FGD), an example of which employs dry sorbent,is generally known in the art. This employs the use of sodiumbicarbonate typically for 10% to 95% sulfur component reduction. Thebicarbonate is initially calcined to sodium carbonate by the flue gasheat, which is typically in the range of 177° C. to 400° C. This thenreacts to form sodium sulfate. Because the sorbent is dry, finely groundpowder, there is no negligible cooling effect with the flue gas and assuch, the stack temperature can be maintained for emission dispersion.Also, the sodium sulfate may be recovered in a baghouse or anelectrostatic precipitator with or without the flyash. The sorbent mustbe processed to a fine particle size, typically 15 μm and then must bestored under dry condition to prevent hold up and enhance manageabilityof the dried sorbent in silos and other equipment.

A further object of the present invention is directed to a process whichcan utilize dry sodium bicarbonate to produce a dry sodium sulfatecompound which can be extracted from gases containing sulfur oxidecompounds.

According to the object of the present invention, there is provided amethod of scrubbing sulfur compounds from gas containing sulfur oxidecompounds in a conditioning vessel, comprising the steps of:

a. providing a sodium sulfate solution;

b. contacting the solution with carbon dioxide and ammonia or ammoniumions while maintaining a solution temperature of at least 32° C. to forma precipitate of sodium bicarbonate;

c. recovering as a product the precipitate of sodium bicarbonate in stepb.;

d. treating the remaining solution of step c with sufficient ammonia orammonium ions to form a saturated solution of ammonium sulfate and aprecipitated sodium sulfate from the solution;

e. washing residual sodium sulfate out of the precipitate of sodiumbicarbonate;

f. dehydrating the precipitate of sodium bicarbonate; and

g. introducing dry sodium bicarbonate into the conditioning vessel forthe removal of a sulfur oxide compound.

This is acceptable for new plants or existing plants with electrostaticprecipitators, baghouses, or other recovery means, but many flue gasdesulfurization (FGD) systems currently operating are wet scrubbingsystems usually using lime as a reagent to reduce sulfur dioxideemissions. One of the primary difficulties is that these systems tend tobe expensive and are plagued with operational difficulties such ascorrosion, slurry handling and disposal problems of products, etc.

A further embodiment of the present invention is directed to a processwhich can utilize a wet scrubbing system and eliminate the corrosionproblems, landfill problems and other handling difficulties associatedwith lime and lime/gypsum slurries.

In accordance with another object of one embodiment of the invention,there is provided a method of scrubbing sulfur compounds from gascontaining sulfur oxide compounds and recovering inorganic values,comprising the steps of:

a. providing a sodium sulfate solution;

b. contacting the solution with carbon dioxide and ammonia or ammoniumions while maintaining a solution temperature of at least 32° C. to forma precipitate of sodium bicarbonate;

c. recovering as a product the precipitate of sodium bicarbonate in stepb.;

d. treating the remaining solution of step c. with sufficient ammonia orammonium ions to form a saturated solution of ammonium sulfate and aprecipitate of sodium sulfate from the solution;

e. recovering as a product the precipitate of sodium sulfate;

f. recovering ammonium sulfate as a product from the remaining solutionfrom step e.;

g. rehydrating the precipitate of sodium bicarbonate to form aconcentrated sodium bicarbonate solution; and

h. introducing the sodium bicarbonate solution into a conditioningvessel for the removal of a sulfur oxide compound.

According to a further object of the present invention, there isprovided a method of scrubbing sulfur compounds from gas containingsulfur oxide compounds, comprising the steps of:

a. providing a source of sodium sulfate solution;

b. contacting the source of sodium sulfate with carbon dioxide andammonia or ammonium ions while maintaining a solution temperature of atleast 32° C. to form a precipitate of sodium bicarbonate;

c. recovering as a product the precipitate of sodium bicarbonate in stepb.;

d. washing residual sodium sulfate out of the sodium bicarbonateprecipitate;

e. converting the precipitate of sodium bicarbonate into a sodiumcarbonate precipitate;

f. rehydrating the sodium carbonate precipitate to form a concentratedsodium carbonate solution; and

g. introducing the sodium carbonate solution into a conditioning vesselfor the removal of a sulfur oxide compound.

The contacting vessel may comprise any vessel suitable for the treatmentof the gas. Examples which are well known include wet scrubbers andspray dryers. It will be appreciated that the conditioning vessel maycomprise at least one of each of the above or a combination of many ofthese.

By integrating the basic bicarbonate recovery process together with thedesulfurization process, the result is a scrubbing process whichadvantageously facilitates recovery of inorganic values, i.e.,fertilizer, sodium carbonate, sodium bicarbonate, etc.

A still further object of the present invention is to provide a methodof scrubbing sulfur compounds from gas containing sulfur oxidecompounds, comprising the steps of:

a. providing a source of sodium sulfate solution;

b. contacting the source of sodium sulfate with carbon dioxide andammonia or ammonium ions while maintaining a solution temperature of atleast 32° C. to form a precipitate of sodium bicarbonate;

c. recovering as a product the precipitate of sodium bicarbonate in stepb.;

d. washing residual sodium sulfate out of the sodium bicarbonateprecipitate;

e. converting the precipitate of sodium bicarbonate into a sodiumcarbonate precipitate; and

f. introducing dry sodium carbonate into a conditioning vessel forremoval of a sulfur oxide compound.

It is well known to those skilled in the art that the Claus process isan effective process for recovering elemental sulfur from hydrogensulfide (H₂ S). The inventive process disclosed herein with respect tothe ammonium sulfate and sodium bicarbonate production can be used incombination with a Claus sulfur plant recovering both liquid sulfur andsulfur dioxide in the form of ammonium sulfate fertilizer and without asulfur plant converting all sulfur components to ammonium sulfatefertilizer.

As will be appreciated by those skilled in the art, the precipitation ofthe above-mentioned precipitates involves exothermic reactions andaccordingly, the heat generated may be recovered for further temperatureregulation in the process. Further, the refrigeration effect nature ofcarbon dioxide gas and ammonia under pressure reduction is useful fortemperature adjustment and regulation in the process, both directly inthe process or indirectly by external means.

Having thus generally described the invention, reference will now bemade to the accompanied drawings illustrating preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the solubility of sodiumbicarbonate, ammonium sulfate and sodium sulfate expressed as a functionof solution temperature;

FIG. 2 is a flow chart illustrating one possible process route foreffecting the method according to the present invention;

FIG. 3 is an alternate embodiment of FIG. 2;

FIG. 4 is an alternate embodiment of FIG. 2;

FIG. 5 is a further alternate embodiment of the process as set forth inFIG. 2;

FIG. 6 is a still further alternate embodiment of the process of FIG. 2;

FIG. 7 is a further alternate embodiment of the process as set forth inFIG. 6;

FIG. 8 is a further alternate embodiment of the process where gypsum isproduced;

FIG. 9 is yet another embodiment of the process according to the presentinvention illustrating a scrubbing process; and

FIG. 10 is an alternate embodiment illustrating how the process can beadapted for use in a wet scrubbing flue gas desulfurization scheme toregenerate sodium bicarbonate from the captured sodium sulfate solution.

Similar numerals in the drawings denote similar elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The chemistry involved according to the present invention can beresolved into the following equations:

    CO.sub.2 +H.sub.2 O=H.sup.+ +HCO.sub.3.sup.-

    NH.sub.3 +H.sub.2 O=NH.sub.4.sup.+ +OH.sup.-

    Na.sub.2 SO.sub.4 +2NH.sub.3 +2H.sub.2 O+2CO.sub.2 =2NaHCO.sub.3 +(NH.sub.4).sub.2 SO.sub.4

Referring now to FIG. 1, shown is a graphical representation of thesolubility curves for sodium bicarbonate, ammonium sulfate and sodiumsulfate. The data are expressed as a function of solution temperature.As is evident from the drawing, the solubility of the bicarbonate andthe sodium sulfate have an overlapping area in which there will be aprecipitation of both of these compounds. As indicated hereinpreviously, this has posed a significant amount of difficulty in theprior art when one was to obtain a substantially pure precipitate ofsodium bicarbonate without the formation of a sodium sulfateprecipitate.

It has been found that if one simply obeys the solubility data, sodiumbicarbonate and ammonium sulfate can be precipitated from a solutioncontaining the molecular species indicated herein above withoutcontamination of one precipitate with the other and further without theprecipitation of the sodium sulfate as an intermediate precipitate.

It has been found that if the sodium bicarbonate crystal and solution ismaintained at a temperature of at least 32° C., under the conditions asset forth with respect to the data in FIG. 1, that the sodiumbicarbonate can be precipitated while the unreacted sodium sulfateremains in solution. If the temperature drops prior to the precipitationof the sodium bicarbonate, the result is that a precipitate of sodiumsulfate solvate or decahydrate will plate out of solution offeringtremendous operating difficulties.

In a chemical system as set forth with respect to the above equations,the system is generally a complex quaternary system, having a reciprocalsalt pair relationship as follows:

    2(NH.sub.4)HCO.sub.3 +Na.sub.2 SO.sub.4 =2NaHCO.sub.3 +(NH.sub.4).sub.2 SO.sub.4

In aqueous solutions above approximately 30° C. ammonium bicarbonate isunstable and dissociates in solution as ions. The subsequentprecipitation of sodium bicarbonate from the system, basically reducesthe system to a complex tertiary system with complications related tohydrate formation and double salt formation. The system and phaseequilibria can be represented on an isothermal diagram which can beemployed to obtain higher purity levels of single components.

The first step in the process is to complete the reaction to drive theequilibrium in the final equation such that the saturated sodium sulfatebrine solution reacts to produce substantially pure sodium bicarbonatecrystals. As is known in the art, numerous possible methods can bepracticed for contacting the ammonia and the carbon dioxide with thesodium sulfate. As an example, the ammonia may be introduced into asolution of the sodium sulfate and carbon dioxide dispersed through thesolution or the carbon dioxide may be dispersed through the saturatedsodium sulfate solution and the ammonia subsequently added or bothcomponents may be dispersed through the solution simultaneously. Anotherpossible alternative includes the use of ammonium bicarbonate.

Referring now to FIG. 2, shown is one possible process route accordingto the present invention. A source of sodium sulfate, such as flyash,for example, from commercial steam boilers containing various levels ofsodium sulfate may be collected from hot flue gas streams andtransferred into a collection silo, globally denoted by numeral 10 inthe drawings. From the silo, the flyash may be transferred at acontrolled rate into an atmospheric mixing container 12, which containeris maintained at a temperature from between about 32° C. and 42° C. Thelight and heavy insolubles are removed in a slurry form from the mixingcontainer 12 at 14. The brine or filtrate is then transferred to aclarifier 16 and further filtered if necessary to polish the solutionfree of fine insolubles. Fine insolubles are removed from the clarifierat 18.

It has been found that one of the main difficulties which previouslyplagued methods practiced in the prior art, was that the temperature ofthe sodium bicarbonate formation reaction was not maintained within theabove-mentioned parameters. The result of this is the formation of ahydrate commonly referred to as Glauber salt (Na₂ SO₄.10H₂ O).Anotherdifficulty which previously plagued methods practices in prior art, wasthe formation of ammonium bicarbonate. It has been found that bymaintaining the temperature within the above-stated range, the Glaubersalt and ammonium bicarbonate does not form and therefore, does notaffect the sodium bicarbonate formation process. In addition, at thistemperature, a maximum amount of salt can be put in solution and therebyreduces the feed circulation rate.

Once the insolubles have been removed by the clarifier 16 at 18, thesolution or brine which contains a small percentage of ammonia is passedinto a first main reactor 20 where the formation of the sodiumbicarbonate occurs. The temperature within the reactor may varydepending on the reactor configuration. The final temperature of thesolution will be progressively reduced to from about 18° C. to about 21°C. with the brine feed temperature to the reactor being maintained above32° C. The final temperature of this solution maximizes bicarbonateyield. This parameter prevents contamination with Na₂ SO₄. Any suitablesolvent may be employed and it will be apparent to those skilled in theart which are suitable possibilities to cover all pressure, temperatureand other operated conditions. Pressure in reactor 20 will preferably bemaintained between 50 to almost 250 psig. This ensures the ammoniaremains dissolved in solution to effect the reaction. A crystallizer maybe included downstream to effect crystallization of the sodiumbicarbonate. Once the crystals have formed, they may be removed from thereactor through a filter means 22 which may comprise a pressure ornon-pressure-type filter. Once the crystals are removed, they may bepassed to a further filtration medium, an example of which may be afiltration screen 24, at which point the formed crystals may be washedwith saturated cold sodium bicarbonate brine or methanol. A high yieldis achievable. The wash may be then returned via line 26 to the mixingcontainer 12. The formed bicarbonate crystals may be then removed fromthe system via line 28 for further uses.

The filtrate or brine from the first reactor is reheated back toapproximately 32° C. The solution is maintained at a temperature of atleast 32° C. and then passed into reactor 32. Once in reactor 32, thebrine solution is subjected with excess ammonium at a concentration ofapproximately 20 weight percent.

The pressure in the reactor is carefully controlled by varying theinjection of ammonia (approximately 490 kPa) thereby controlling thedesired concentration of excess ammonium. In reactor 32, the injectionof the solution with ammonia shifts the equilibrium solubility of thesolution of the reaction, denoted hereinabove, to favour the formationof ammonium sulfate precipitate. The temperature in the reactor ismaintained at 32° C. to keep free sodium cations soluble and thereforeto prevent contamination of the ammonium sulfate with undesirablesolvates. When desired, the ammonia concentration can be altered bychanging the pressure control. Similar to the description for reactor20, reactor 32 may include a crystallizer downstream to effect theformation of ammonium sulfate crystals. Once formed, the crystals may bepassed onto a pressure filter medium 34 and washed with saturated coldammonium sulfate brine wash. The so-formed ammonium sulfate crystals canthen be removed by line 36 for further uses. The wash solution may bereturned to the mixing container 12 via line 38 for further uses. Theammonia containing filtrate remaining after the precipitation of theammonium sulfate crystals, may be flashed off, compressed and condensedand collected in to a surge drum 40 as is known in the art. Oncecollected, the ammonia solution may be used for reinjection in thesystem.

The final recovered solution, containing soluble levels of ammonia canbe recycled to the mixing container 12 to complete the continuousoperation.

By practicing the above method, a purity of ammonium sulfate greaterthan 50% by weight is achievable.

Advantageously, the ammonia can be substantially recovered for reusewhich has positive economic advantages for the entire process.

FIG. 3 shows a further variation on the process according to FIG. 2. InFIG. 3, a brine conditioning step is employed between reactors 20 and32. The brine conditioning step is effective to purify the feed streamfor introduction into reactor 32 for eventual formation of ammoniumsulfate by the further reduction of sodium ion concentration from thefeed stream entering into reactor 32.

Once the sodium bicarbonate reaction has been completed, the bicarbonateprecipitate is removed as set forth herein with respect to FIG. 2 andthe brine is transferred to intermediate reactor 42. In reactor 42, theconcentration of the ammonia is increased to saturate the solution whilethe temperature of the reactor is lowered to approximately 7° C. Thisresults in the formation of a precipitate comprising either pure sodiumsulfate, or a mixed precipitate of sodium sulfate and ammonium sulfate.These precipitates are then filtered by filter 44 and the crystalseventually passed back into contact with mixing container 12. Thefiltrate is then fed to reactor 32, maintained under at least the samepressure conditions as indicated for FIG. 2. Once in reactor 20, thefiltrate undergoes the reaction as indicated herein above, the result isthe formation of ammonium sulfate precipitate, however, the precipitateis formed in an environment where the sodium cation concentration issignificantly reduced in view of the intermediate process usingintermediate reactor 42. The result of the process is a solutionconcentration of a ammonium sulfate which will effect a precipitate of aconcentration greater than 73% by weight.

Referring now to FIG. 4, shown is a further alternate arrangement bywhich the process may be practiced. In FIG. 4, the overall process mayinclude a separate washing step for washing the sodium bicarbonate andammonium sulfate precipitates separately. In one possible configuration,the sodium bicarbonate which is formed in reactor 20, may be passed intocontact with a washing material, an example of which may be a source ofmethanol 50. The resulting filtrate may then be returned to mixingcontainer 12 via line 52.

Similarly, the ammonium sulfate crystals formed in reactor 42, may bepassed through a second independent source of methanol 54 with thefiltrate being returned to mixing container by line 56. The ammoniumsulfate crystals and bicarbonate can then be used for further uses.

Although the process as discussed herein has been indicated to beprimarily conducted in water, it will be understood by those skilled inthe art that any suitable solvent can be used provided the choice ofsolvent does not vary the solubility relationship necessary to effectthe process. As one possible alternative, glycol may be employed as thesolvent.

Referring now to FIG. 5, shown is a further variation on the schematicprocess shown in FIG. 1. In the process shown in FIG. 5, the filtraterecovered from the sodium bicarbonate reaction can be made to be acommercially substantially pure liquid product, e.g. a fertilizer in thenear saturated state. This affords the user the opportunity of blendingthe liquid product wits crystallization of the product in the desiredform. As is illustrated in FIG. 5, the liquid product may be passed fromreactor 20 to the brine conditioning container 42 where the temperatureof the ammonia sulfate solution is reduced to approximately 7° C. as setforth herein previously with respect to FIG. 3. In this embodiment, theammonia concentration is increased from about 0% to about 50% or greaterby weight to therefore provide a supersaturated solution. The result isthe precipitation of sodium sulfate or mixed salts of ammonium andsodium sulfate. The filtrate in this situation is substantiallysaturated liquid ammonium sulfate which can then be passed on to astorage unit 60. As a further alternative, a user may simply pick up theliquid ammonium sulfate or alternatively, the ammonium sulfate may bepumped into a conventional evaporator (crystallizer) 62 which wouldafford the user the opportunity to mix the liquid with additionalfertilizer components etc. and have the final product crystallized.

The brine conditioning can be performed in a single step or it may beconditioned in multiple steps to achieve increased removal of sodiumcations; this inherently leads to increased purity of the ammoniumsulfate fertilizer. The above-mentioned steps can be any combination ofknown (salting out) steps i.e. evaporation, addition of excess ammonia,etc.

FIG. 6 shows a variation on the process where the bicarbonate recoverysystems as set forth herein previously can be combined to be useful in asulfur recovery plant. Generally speaking, the area designated bynumeral 100 in FIG. 6 illustrates conventional apparatus employed forsulfur recovery from an acid gas stream by employing the modified Clausreaction, consisting of a single or multiple variation of thermal andcatalytic recovery steps.

It is well known to those skilled in the art that the Claus process isuseful for desulfurization. Generally speaking, the process is effectedin two steps, namely: ##EQU1## This generally results in a sulfurrecovery of approximately 90% to 96% in a liquid sulfur state. Theremaining sulfur containing component is recovered in sulfur recoverytechniques such as Tail Gas units. By employing the recovery process asset forth herein previously, sodium bicarbonate can be introduced intothe tail gas stream containing residual sulfur compounds and results cantherefore be the production of ammonium sulfate as indicated in FIG. 6.As is illustrated in FIG. 6, the overall modified Claus process, denotedby numeral 100 can be combined with the overall process for producingammonium sulfate, the group of steps of which is generally indicated bynumeral 115 in the figure. The broad steps as illustrated in the figureare generally common steps to those shown in FIGS. 2 and 3. By combiningthe modified Claus process with the processes as set forth herein, theresult is sulfur removal of the order of at least 95% or greater.

Turning to FIG. 7, shown is a variant on the process schematicallyillustrated in FIG. 6, but for a lower volume production sulfur plant,typically having production levels of less than 10 MTD where economicconstraints preclude the system shown in FIG. 6. The steps for theprocess are similar to those for FIG. 6 and the treatment of the sulfurcompound is generally denoted by the sequence of events as indicated bynumeral 115.

The acid gas stream may be as an alternative directly treated withliquid sodium bicarbonate or carbonate solution for desulfurization andform an alternate sulfur product.

Turning to FIG. 8, showing schematically is a further embodimentaccording to the present invention. The embodiment shown, a lime mixingcontainer 70 is provided for retaining lime in any form, e.g. a slurryor powder form to be introduced into reactor 32 via line 72. Byproviding this addition to the recovery unit, commercial or landfillgypsum can be produced along with sodium bicarbonate as illustrated inthe figure, the arrangement shown may include ammonia recovery unit 74which will include the usual gaseous recovery means well known to thoseskilled in the art. This is useful since the ammonia is liberatedsubsequent to precipitation of gypsum and therefore can be easilyrecovered.

With more specific reference to FIG. 9, there is illustrated aneffective scheme whereby the recovery process as set forth herein, canbe employed to regenerate dry sodium sulfate captured from a particulatecollection device 150 such as an electrostatic precipitator or baghouseand produce sodium bicarbonate to be injected into the flue gas streamto primarily reduce sulfur components from sulfur sources such as anindustrial boiler 180 and released by means of a stack 152. The practiceof injecting a dry chemical into the hot flue gases is commonly referredto as DSI. By adapting the recovery process, discussed with respect toFIG. 3, to the DSI technique, the overall scheme becomes a continuousregenerative process with no waste streams or landfill requirements, noappreciable losses, and all by-products are immediately commerciallyuseable. Using dry sodium bicarbonate as the reagent offers theadditional advantages of further recovery of other undesirablecomponents such as NO_(X), HCl and SO₃ from the flue gases, and improvethe performance of the downstream collection device 150 (i.e. ESP orbaghouse). The dry sorbent technique, in addition, will not appreciablyaffect the temperature of the flue gases, thereby maintaining orimproving the effluent emission dispersion from the stack 152.

As discussed previously, the sodium sulfate containing flyash compoundis transferred from the collection device, stored in a silo 10 andblended into the feed preparation tank of the process denoted by 115(FIGS. 6 and 7). The feed is processed in the bicarbonate recoverysystem 122 and the sodium bicarbonate is passed into a dry sorbentinjection unit 126 and reintroduced into the system via line 128.By-product from the bicarbonate recovery unit 122 is transferred to thefertilizer recovery unit 124, the resulting product is commercial gradefertilizer. It will be appreciated that the process can be easilyconducted under wet conditions as will as be appreciated.

Turning to FIG. 10, shown is a further variation on the overall processaccording to the present invention. In FIG. 10, a flue gasdesulfurization (FGD) process using a wet sodium bicarbonate injectionsystem for desulfurization, employs sodium carbonate or bicarbonate asthe active reagent.

In the embodiment illustrated, flue gas from the industrial boiler ortail gas unit, represented by numeral 180, is passed into anelectrostatic precipitator or baghouse 182 or other recovery device toremove flyash at 184. A water wash container 186 is provided tocirculate wash water in the upper section of the scrubber 188 andaccumulated levels of precipitates and fluids are drawn off fromcontainer 186 and passed to the lower section of the scrubber 188. Oncesodium sulfate is collected from the bottom of scrubber 188 as a productof the scrubbing procedure, it is then further transferred to mixingcontainer for thickening and clarification to a saturated state forfeeding into bicarbonate recovery apparatus 122. From apparatus 122sodium bicarbonate is filtered from the solution and washed in eitheropen screen, pressure type, vacuum type, centrifuge or cyclone typefilters or any combination of these. The bicarbonate precipitate iswashed and reduced to less than 10% liquid and then fed as a slurry intoa bicarbonate slurry container 96 at approximately 700 kPa. At thispoint, the bicarbonate slurry in container 96 is mixed with clean boilerfeed water supplied to container 96 from a feed water supply container98. The feed water is maintained at a temperature of approximately 48°C. The slurry is continually mixed and ranges in a concentration ofbetween about 20% by weight to about 40% by weight. The slurry is thentransferred to a high pressure solution container 200 at a pressure ofapproximately 1050 kPa, where a saturated solution is formed. Asaturated bicarbonate solution is created using additional boiler feedwater from container 98 which is heated to approximately 176° C. by aninjection water heater 102. The final saturated concentrated solution isthen injected into wet scrubber 188 for sulfur dioxide removal via line201.

It will be appreciated by those skilled in the art, that sodiumcarbonate can be used as a replacement to sodium bicarbonate. Theconversion can easily be accomplished by, for example, calcining thebicarbonate in a dry form or by increasing the temperature in a liquidform and recovering the carbon dioxide for recycle to the bicarbonaterecovery apparatus 122. Other suitable methods will be apparent to thoseskilled. The ammonia used in the process can be recovered in a recoveryprocess as set forth herein with respect to other embodiments and thisis equally true of the ammonium sulfate and other compounds in theprocess.

The temperature, pressure and concentration of reagent in the finalinjection solution can be varied to control the level of SO₂ removed andthe final flue gas temperature exiting the wet scrubbing process. As afurther example, the temperature and pressure can be reduced to nearatmospheric conditions prevalent in the scrubber 188. The temperaturecan be reduced to 99° C. to eliminate water heater 102 and the highpressure reactor 200. This will result in a cooler final flue gastemperature resulting from the evaporative cooling effect which may ormay not be detrimental to any specific application.

In addition, it will be appreciated by those skilled in the art, thatthe wet scrubber 188 can take any form of contacting the reactantsolution with the sulfur containing flue gas, for example spray driers,etc.

It will be readily appreciated by those skilled that the solubilityshift discussed herein can be effected by regular evaporation, or by theaddition of any suitable compound which provides a salting effectwithout effecting the chemical composition of the desired product salts.

As a consequence of reactor vessel size, temperature stratification mayexist within the reactors as set forth herein or the crystallizingvessels to enhance the crystal growth, stability and yield. In order toavoid undesirable effects caused by hydrate or solvate precipitation,the process can be performed in multiple vessels to circumvent thesedifficulties.

Although embodiments of the invention have been described above, it isnot limited thereto and it will be apparent to those skilled in the artthat numerous modifications form part of the present invention insofaras they do not depart from the spirit, nature and scope of the claimedand described invention.

We claim:
 1. A method of scrubbing sulfur compounds from gas containingsulfur oxide compounds and recovering inorganic values, comprising thesteps of:a. providing a sodium sulfate solution; b. contacting saidsolution with carbon dioxide and ammonia or ammonium ions whilemaintaining a solution temperature of at least 32° C. to form aprecipitate of sodium bicarbonate; c. recovering as a product saidprecipitate of sodium bicarbonate in step b.; d. filtering precipitatedsodium bicarbonate; e. treating the remaining solution of step c. withsufficient ammonia of ammonium ions to form a saturated solution ofammonium sulfate and a precipitate of sodium sulfate from said solution;f. recovering as a product said precipitate of sodium sulfate; g.recovering ammonium sulfate as a product from the remaining solutionfrom step e.; h. rehydrating said precipitate of sodium bicarbonate toform a concentrated sodium bicarbonate solution; and i. introducing saidsodium bicarbonate solution into a conditioning vessel for the removalof a sulfur oxide compound.
 2. The method as set forth in claim 1,wherein said method further includes the steps of:washing residualsodium sulfate out of said precipitate of sodium bicarbonate; anddehydrating said precipitate of sodium bicarbonate.
 3. The method as setforth in claim 2, wherein said step of dehydrating said sodiumbicarbonate comprises dehydrating said sodium bicarbonate precipitate toless than about 10% by weight liquid.
 4. The method as set forth inclaim 1, wherein said gas is flue gas.
 5. The method as set forth inclaim 4, further including the step of removing flyash from said fluegas.
 6. The method as set forth in claim 5, further including the stepof removing flyash with an electrostatic precipitator or a baghouse. 7.A method of scrubbing sulfur compounds from gas containing sulfur oxidecompounds in a conditioning vessel, comprising the steps of:a. providinga sodium sulfate solution; b. contacting said solution with carbondioxide and ammonia or ammonium ions while maintaining a solutiontemperature of at least 32° C. to form a precipitate of sodiumbicarbonate; c. recovering as a product said precipitate of sodiumbicarbonate in step b.; d. filtering precipitated sodium bicarbonate; e.treating the remaining solution of step c. with sufficient ammonia orammonium ions to form a saturated solution of ammonium sulfate and aprecipitated sodium sulfate from said solution; f. washing residualsodium sulfate out of said precipitate of sodium bicarbonate; g.dehydrating said precipitate of sodium bicarbonate; and h. introducingdry sodium bicarbonate into said conditioning vessel for the removal ofa sulfur oxide compound.
 8. The method as set forth in claim 7, furtherincluding the step of collecting dry sodium sulfate for introductioninto said sodium sulfate solution in step a.
 9. The method as set forthin claim 7, further including the step of recycling said carbon dioxide.10. The method as set forth in claim 7, further including the step ofthickening and clarifying said sodium sulfate prior to step b.
 11. Themethod as set forth in claim 10, wherein said step of dehydrating saidsodium bicarbonate comprises dehydrating said precipitated sodiumbicarbonate to less than about 10% by weight liquid.
 12. The method asset forth in claim 7, wherein said gas is flue gas.
 13. A method ofscrubbing sulfur compounds from gas containing sulfur oxide compounds,comprising the steps of:a. providing a source of sodium sulfatesolution; b. contacting said source of sodium sulfate with carbondioxide and ammonia or ammonium ions while maintaining a solutiontemperature of at least 32° C. to form a precipitate of sodiumbicarbonate; c. recovering as a product said precipitate of sodiumbicarbonate in step b.; d. filtering precipitated sodium bicarbonate; i.converting said precipitate of sodium bicarbonate into a sodiumcarbonate precipitate; and j. introducing dry sodium carbonate into aconditioning vessel for removal of a sulfur oxide compound.
 14. Themethod as set forth in claim 13, further including the step of recyclingcarbon dioxide.
 15. The method as set forth in claim 13, wherein saidstep of converting said sodium bicarbonate to sodium carbonate comprisescalcining said sodium bicarbonate precipitate.
 16. The method as setforth in claim 13, wherein said step of converting said sodiumbicarbonate to sodium carbonate comprises elevating the temperature ofthe sodium bicarbonate precipitate in solution.
 17. The method as setforth in claim 13, further including the step of filtering thickeningand clarifying said sodium sulfate prior to step b.
 18. The method asset forth in claim 17, further including the step of filtering saidprecipitated sodium carbonate prior to step i.
 19. The method as setforth in claim 13, wherein said gas comprises flue gas.
 20. The methodas set forth in claim 19, further including the step of removing fly ashfrom said flue gas.
 21. The method as set forth in claim 20, furtherincluding the step of removing fly ash with an electrostaticprecipitator or baghouse.
 22. A method of scrubbing sulfur compoundsfrom gas containing sulfur oxide compounds, comprising the steps of:a.providing a source of sodium sulfate solution; b. contacting said sourceof sodium sulfate with carbon dioxide and ammonia or ammonium ions whilemaintaining a solution temperature of at least 32° C. TO form aprecipiate of sodium bicarbonate; c. recovering as a product saidprecipitate of sodium bicarbonate in step b.; d. filtering precipitatedsodium bicarbonate; e. treating the remaining solution of step c. withsufficient ammonia or ammonium ions to form a saturated solution ofammonium sulfate and a precipitate of sodium sulfate from said solution;f. recovering as a product said precipitate of sodium sulfate; g.recovering ammonium sulfate as a product from the remaining solution ofstep f.; h. washing residual sodium sulfate out of said sodiumbicarbonate precipitate; i. converting said precipitate of sodiumbicarbonate into a sodium carbonate precipitate; and j. introducing drysodium carbonate into a conditioning vessel for removal of a sulfuroxide compound.